342-3 Effects of Cations on Structural Transformation of Layered Manganese Oxides during Oxidation of Fulvic Acid.
Wednesday, October 25, 2017: 8:35 AM
Tampa Convention Center, Room 37
Partially reductive dissolution of layered manganese oxides (birnessite) enriches the oxide structure with trivalent manganese (Mn(III)). Naturally occurring birnessite contains abundant metal cations. We examined the effects of the presence of Zn2+, Mg2+ or Ca2+ and ionic strength (0 mM versus 50 mM NaCl) on the structural transformation of birnessite (d-MnO2) by reaction with fulvic acid (FA) at pH 8 and FA/MnO2 mass ratios (R) of 0.1 or 1 over 25 d under anoxic conditions. Both the higher ionic strength and the presence of the divalent cations enhanced production of dissolved Mn and more strongly, FA adsorption and fractionation. Both Mg2+ and Ca2+ favor the Mn(III) formation in the layers and on the vacancies of reacted d-MnO2 while Zn2+ shows an opposite effect. The presence of these cations also results in less Mn(II) adsorbed on vacancies. In addition, minor feitknechtite and hausmannite formed in the presence of Ca2+ or Mg2+. In the presence of Zn2+, no new phases formed at R = 0.1; however, at R = 1, Zn-substituted hausmannite formed extensively at the late stage. Our results indicate that the Mn(III) enrichment in layered Mn oxides during partial reductive disolution follows the same mechanism as that during biogenic oxidative precipitaton, i.e., the comproportionation between vacancy‑adsorbed Mn(II) and Mn(IV) in the layers. Ca2+, Mg2+ and high ionic strength all stabilize Mn(III) in the layers while Zn2+ competes with Mn(II) for vacancies, respectively enhancing and decreasing Mn(III) production. This work also highlights that Mn oxides can effectively stabilize organic carbon in the Mn-rich sediments of salty environments.